The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures

Study of the Ternary Mixture of Methanol/Formamide/Acetonitrile via Solvatochromic Probes

Following previous studies, the ternary mixture of methanol/formamide/acetonitrile (MeOH/Formamide/MeCN) was studied using the UV-Vis absorption spectra at 298.15 K with a set of five probes, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, N,N-dimethyl-4-nitroaniline and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (Reichardt betaine dye), for a total of 22 mole ternary fractions. In addition, nine mole fractions of the underling binary mixtures, MeOH/Formamide and Formamide/MeCN were also tested. Spectroscopic results were used to model the preferential solvation order for each probe in the mixtures. The Kamlet-Taft solvatochromic solvent parameters, α, β, and π*, were also computed through the use of the solvatochromic shifts of the five probe indicators. Moreover, discrepancies in the spectroscopic behavior of 4-nitrophenol in formamide-rich mixtures were observed and analyzed.

3-aminoquinoline: a turn-on fluorescent probe for preferential solvation in binary solvent mixtures

3-Aminoquinoline (3AQ) has been used as a fluorescent probe for preferential solvation in hexane-ethanol solvent mixtures. Results of the present experiment have been put into context by comparison with prior observations with 5-aminoquinoline (5AQ) as the probe. 3AQ exhibits a relatively small change of dipole moment (Δμ= 2.2 D) upon photoexcitation, compared to 5AQ (Δμ= 6.1D), which might appear to be a hindrance in the way of its use as a solvation probe. Indeed, the values of parameters like spectral shifts are smaller for the present experiment with 3AQ. At the smallest concentration of alcohol used, its local mole fraction around the probe is significantly lower than in the previous experiments with 5AQ. However, these apparent disadvantages are outweighed by the significant increase in fluorescence intensity and lifetime observed with increasing concentration of ethanol in the solvent mixture, as opposed to the drastic fluorescence quenching that occurs for 5AQ. This is a marked advantage in the use of 3AQ in studies like the present one. The local mole fraction of ethanol and preferential solvation index experienced by 3AQ are in line with those reported for 5AQ. The disadvantage of the smaller magnitude of Δμpersists in the time resolved fluorescence experiments, for solvent mixtures with very low ethanol content. Negligible wavelength dependence of fluorescence transients of 3AQ is observed forx_p= 0.002,. However, this effect is outweighed at higher alcohol concentrations, for which nanosecond dynamics of preferential solvation is observed.

Development of a carbon dot and methylene blue NIR-emitting FLIM-FRET pair in niosomes for controlled ROS generation

Non-ionic surfactant vesicular systems (niosomes) are structurally similar to lipid vesicles, differing only in the bilayer composition. Herein we report a unique method to generate reactive oxygen species (ROS) utilizing a FLIM-FRET technique involving niosome-trapped yellow emissive carbon dots (YCDs) and methylene blue (MB) in aqueous medium under neutral conditions. Niosomes are biologically important because of their good stability and extremely low toxicity. Fluorescent CDs, emitting in the higher wavelengths on visible light excitation, are of incredible importance in bio-imaging and optoelectronics. Hence, we prepared nitrogen-containing YCDs from a single precursor, o-phenylenediamine, and explained their detailed photophysics upon incorporation into the niosomal bilayer. The YCDs are polarity sensitive, and are rotationally restricted in niosomes, which increases their fluorescence quantum yield from 29% (in water) to 91%. These YCDs are tactically employed to develop a near infrared (NIR) FRET pair with methylene blue (MB), which is a very well-known type-I and type-II photosensitizer. This FRET pair, which emits in the NIR region, is found to be an ideal system to generate ROS by excitation in the lower visible wavelengths. Interestingly, the ROS production by MB from the dissolved oxygen is enhanced inside the niosomes. The donor and the acceptor moieties in this unique NIR-emitting FRET pair display an unprecedented 300 nm Stokes shift. The findings could be influential in bio-imaging in the NIR region evading cellular autofluorescence and the controllably generated ROS can be further applied as a potential photodynamic therapeutic agent.